Not all archs have io memory.
Instead of selecting I2C_MUX (and bypassing the HAS_IOMEM dependency)
depend directly on it.

Fixes the following kconfig warning:
warning: (MEDIA_SUBDRV_AUTOSELECT && VIDEO_CX231XX && INV_MPU6050_I2C) selects I2C_MUX which has unmet direct dependencies (I2C && HAS_IOMEM)
And this build error:
ERROR: "devm_ioremap_resource" [drivers/i2c/muxes/i2c-mux-reg.ko] undefined!
ERROR: "of_address_to_resource" [drivers/i2c/muxes/i2c-mux-reg.ko] undefined!

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>